Photodiodes convert light into electrical signals, and form the basis of a wide range of imaging and detection devices. Common configurations of photodiodes include PIN photodiodes, operated in photovoltaic or photoconductive mode, and single photon avalanche diodes (SPADs), typically operated in Geiger mode. Photodiodes can be realised using a number of different technologies, but it is particularly convenient and economical to form photodiodes using the CMOS fabrication process that has evolved as the production method of choice for integrated circuits of almost all types.
Some examples of a PIN photodiode formed in a CMOS fabrication process are given in Ciftcioglu et al, “Integrated Silicon PIN Photodiodes Using Deep N-Well in a Standard 0.18-μm CMOS Technology,” Journal of Lightwave Technology, Vol. 27, No. 15, 1 Aug. 2009.
FIG. 1 is a schematic of a known lateral PIN photodiode, corresponding to the device shown in FIG. 1(c) of the Ciftcioglu et al document. During an epi-CMOS fabrication process, a lightly doped p-type epi-layer 102 is grown on a more heavily doped P-type substrate 100. Two n-wells (containing n-type doping) 104, 106 are formed in the epi-layer 102, and cathodes 108, 110 and an anode 112 are deposited on the surface, with highly doped n-type and p-type regions beneath the contacts. During operation, photons striking the region between the n-wells 104, 106 and the anode create charge carriers, either causing current to flow or voltage to build up between the contacts depending on the mode of operation.
(Within this document, terms such as ‘above’, ‘on’, ‘below’, ‘top’, ‘bottom’, ‘horizontally’ and ‘vertically’ should be construed with reference to the integrated circuit cross-sections illustrated in the attached figures. In particular, a horizontal direction runs parallel to the substrate, and a vertical direction runs perpendicularly in and out of the substrate, with ‘down’ running deeper into the substrate, and ‘up’ leading to the surface, or ‘top’ of the device. Accordingly, epitaxial growth and component formation during the fabrication process will proceeds in an ‘upwards’ direction to the ‘top’ of the device.)
FIG. 2 is a schematic of another known lateral PIN photodiode, corresponding to the device shown in FIG. 1(d) of the Ciftcioglu et al document. Again a lightly doped p-type epi-layer 202 overlies a more heavily doped p-substrate 200. In this device, a ‘deep n-well’ 204 is formed in the epi-layer 202. A p-well 206 is formed above the deep n-well 204 and the n-wells 208, 210 and cathodes 212, 214 and anode 216 are formed as before.
The deep n-well feature is a standard part of the CMOS fabrication toolkit available to circuit designers, and is conventionally used to allow transistor isolation to be improved and to reduce substrate noise coupling in mixed-signal and RF circuits. Deep n-wells are formed using ion implantation to form an n-type region deep in the substrate or epi-layer. The deepest part of a deep n-well may for example be in the region of 2 μm below the surface of the wafer. In the device of FIG. 2, the deep n-well allows for a reduction in size of the depletion region at the top of the deep n-well, allowing increased bandwidth with a reduction in bias voltage.
Considering now a single photon avalanche diode, SPAD (which is operated in a different mode to PIN photodiodes), one requirement for a SPAD to effectively operate above breakdown is that there should be no high field localisation around the edge of the detector active area. This criterion can be met by using a ‘guard ring’, which can be fabricated by a variety of methods. A common feature of such methods is a structure that successfully raises the breakdown voltage of the periphery of the active region of the detector above the breakdown voltage of the planar, or light sensitive, region.
FIG. 3 is a schematic of a known SPAD, described for example in Rochas et al, “Single photon detector fabricated in a complementary metal-oxide-semiconductor high-voltage technology,” Review of Scientific Instruments, Vol. 74, No. 7, 2003. In FIG. 3, a deep n-well 302 is formed in a substrate 300, and heavily doped n-type and p-type doped regions are created to form a dual p+/deep n-well/p-substrate junction. A p-well guard ring 304 is also formed, and a metal ring 306 is located above the p+ anode with a central gap of approximately 7 μm. The upper p+/deep n-well junction provides the multiplication region where Geiger breakdown occurs. However, one problem with the device shown in FIG. 3 is that very high doping concentrations on both sides of the active junction lead to a high electric field, which results in excessive noise due to band-to-band tunnelling.
Other guard ring constructions have been demonstrated to work with SPADs, such as Shallow Trench Isolation (STI). On example of a STI guard ring is given in WO 2008/011617, for example. STI works as a guard ring because its permittivity is lower than silicon, allowing it to dissipate high electric fields successfully. However, traps at the STI/silicon interface mean that such devices have high parasitic count rates.
A problem suffered by known CMOS SPADs is that their peak detection efficiency is at blue light wavelengths, because the active region of the devices is relatively shallow (with respect to the surface). Electromagnetic energy has different characteristic absorption depths in silicon depending on its wavelength. High frequency, high energy, short wavelength blue light is absorbed close to the surface, while long wavelength red and near infra-red (NIR) is absorbed deeper. Statistically it is more probable that a red photon will generate an electron deeper in the silicon than a blue photon.
Red and NIR response is a particularly important feature for SPADs because of two main application areas: range detection and lifetime analysis. NIR frequencies of 850 nm and 940 nm are commonly used in ranging systems because many LEDs are available at 850 nm, and 940 nm corresponds to a window in solar output leading to increased signal-to-noise ratio operating outdoors. Moreover, SPADs have been used in biological experiments for lifetime estimation. Since blue corresponds to high energy photons, it has the disadvantage of phototoxicity for the cells which are to be observed. Moreover, near-infra red will penetrate further into tissue with potential applications in diffuse optical tomography (DOT). A third important application is the use of SPADs in communications where near-infra red is commonly employed because it is efficiently transmitted within optical fibres.
The present invention seeks to address these and other problems in the prior art.